Epidemiological studies consistently report that one of the most reliable predictors of whether an adolescent or young adult will use drugs is whether or not his or her friends use drugs. The reasons for the high concordance rate of substance use among members of peer groups are not fully known, but two types of theories have received the most attention. Selection theories suggest that individuals self-select into social groups that are similar to themselves, whereas socialization (or social-learning) theories propose that members of a social group model substance use behaviors and other group members imitate those behaviors. Experimental studies examining the role of social influence on drug self-administration in humans are limited because ethical constraints limit the degree to which substance use can be modeled. Experimental studies in animals are limited because subjects are typically removed from their home environment and separated from their cagemates during testing. This represents a significant limitation of animal models, because drug use in humans typically occurs in the presence of others, and the behavior of these other people (i.e., whether or not they are also using drugs) may influence the drug consumption of the individual. The proposed project describes the use of custom-built operant-conditioning chambers that permit intravenous drug self- administration to be examined in multiple animals at the same time and in the same chamber. In a series of experiments, we will test whether animals choose other rats that share a similar self- administration history (as selection theories predict) and whether rats imitate the self-administration behavior of a peer (as socialization theories predict). Because socialization theories further predict that interventions targeting the social environment will influence drug self-administration, we will examine the impact of having a companion that also self-administers cocaine (i.e., a peer user) versus the impact of having a companion that does not self-administer cocaine (i.e., a peer nonuser). Our central hypothesis is that cocaine self-administration will be increased in rats paired with a companion with access to cocaine and decreased in rats paired with a companion without access to cocaine. Also, given the high scientific priority that NIDA places on investigating sex differences in preclinical and clinical research, all studies will employ both male and female subjects.